In general, a solid electrolytic capacitor is formed through the following steps: a dielectric oxide film layer is formed on a positive electrode substrate formed of a metallic foil which undergoes etching treatment and has a large specific surface area; a solid semiconducting layer (hereinafter referred to as “solid electrolyte layer”) serving as a counter electrode is formed outside the oxide film layer; preferably a conducting layer comprising metallic powder or a conducting layer comprising a conductive carbon layer and a layer formed thereon comprising metallic powder is further formed on the outer side of the solid electrolyte layer; and a lead wire is connected thereto, thereby forming the basic elements of a capacitor. Subsequently, the entirety of the elements is completely sealed by use of an epoxy resin or the like. The thus-obtained product is widely used as a capacitor component in electric appliances.
In recent years, in order to meet requirements for digitization of electric apparatuses and increase in processing speed of personal computers, solid electrolytic capacitors are demanded to have small size, high capacitance, and low impedance in a high-frequency range.
In order to meet demands for such solid electrolytic capacitors, suggestions have been made with regard to solid electrolytes, conducting materials, etc.
For the solid electrolyte, it is heretofore known to use, for example, an inorganic semiconductor material such as manganese dioxide and lead dioxide, an organic semiconductor material such as TCNQ complex salt, an intrinsic electrically conducting polymer having an electric conductivity of from 10−3 to 5×103 S/cm (JP-A-1-169914 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) corresponding to U.S. Pat. No. 4,803,596) or an electrically conducting polymer such as π-conjugated polyaniline (JP-A-61-239617), polypyrrole (JP-A-61-240625), polythiophene derivative (JP-A-2-15611, U.S. Pat. No. 4,910,645) or polyisothianaphthene (JP-A-62-118511).
Capacitors using manganese dioxide for the solid electrolyte are disadvantageous not only in that when manganese nitrate is thermally decomposed to form manganese dioxide, the oxide dielectric film once formed on the anode foil is ruptured, but also in that the impedance property is not satisfied.
In the case of using lead dioxide, cares on the environment are additionally required.
Capacitors using a TCNQ complex salt solid for the solid electrolyte have good heat molten workability and excellent electric conductivity but are considered to show poor reliability in the heat resistance at the solder joining (soldering heat resistance) because the TCNQ complex salt itself has a problem in the heat resistance.
Capacitors using an electrically conducting polymer for the solid electrolyte are free of rupture of dielectric film and favored with high impedance property but disadvantageously deficient in the heat resistance, thermal shock resistance and vibration resistance.
With respect to the method for forming a solid electrolyte using an electrically conducting polymer, for example, a method of melting an electrically conducting polymer (solid electrolyte) as described above on a dielectric film layer on the surface of a valve-acting metal having fine void structures to form an electrically conducting polymer layer, and a method of depositing the above-described electrically conducting polymer on the dielectric film layer are known.
More specifically, in the case of using, for example, a polymer of a 5-membered heterocyclic compound such as pyrrole or thiophene for the solid electrolyte, there are known a method of forming an electrically conducting polymer layer having a necessary thickness by repeating a series of operations of dipping an anode foil having formed thereon a dielectric film in a lower alcohol and/or water-based solution of a 5-membered heterocyclic compound monomer and after pulling it up, again dipping the foil in an aqueous solution having dissolved therein an oxidizing agent and an electrolyte to cause chemical polymerization of the monomer (JP-A-5-175082), a method of coating simultaneously or not simultaneously a 3,4-ethylenedioxythiophene monomer and an oxidizing agent each preferably in the form of a solution on the oxide film layer of a metal foil to form an electrically conducting polymer layer (JP-A-2-15611 (U.S. Pat. No. 4,910,645) and JP-A-10-32145 (European Patent Laid-Open Publication 820076)), and the like.
As the oxidizing agent for use in conventional techniques, for example, chemical polymerization of 5-membered heterocyclic compounds such as thiophene, there are known iron(III) chloride, Fe(ClO4)3, organic acid iron(III) salt, inorganic acid iron(III) salt, alkyl persulfate, ammonium persulfate (hereinafter simply referred to as “APS”), hydrogen peroxide, K2Cr2O7, etc., (JP-A-2-15611), cupric compounds, silver compounds, etc., (JP-A-10-32145 (European Patent Laid-Open Publication 820076)).
In recent years, a method for producing a polyaniline composite is proposed, where powdered polyaniline is used as an electrically conducting starting material, rubber and/or thermoplastic resin is used as the matrix material and the powdered polyaniline is dispersed and compounded in the rubber and/or thermoplastic resin to form a polyaniline composite having mechanical strength and flexibility (JP-A-64-69662).
Furthermore, a method for producing a capacitor is proposed, where a composite film is formed on the metal oxide of a capacitor electrode from a polyaniline solution containing from 1 to 25 mass % of a polymer binder and an electrically conducting polymer layer comprising polyaniline having added thereto anion is further formed on the composite film (JP-A-5-3138).
According to the above-described methods, it is necessary for forming an electrically conducting polymer layer to previously form a thin electrically conducting layer on the oxide film as an insulator by chemical polymerization. Furthermore, there are problems mentioned below in suitability applying these methods to respective capacitors.
First, in the case of electrolytic polymerization, if the polymer has poor flexibility, the increase in viscosity causes reduction in capacitance. More specifically, when an aluminum foil having formed thereon a dielectric material obtained by etching the surface is dipped with an oxidizing agent solution and then dried, an oxide film having high viscosity is formed on the surface of a porous body. As a result, microfine pore openings present on the surface of the porous body are clogged. Furthermore, a polymer is formed on the surface by the contact with a monomer and the polymer is not formed inside the pores, which causes reduction in capacitance.
Second, in the case of chemical polymerization, the amount of polymer adhered by one polymerization step is small, accordingly, the dipping must be repeated with predetermined number of steps. Thus, a method advantageous in view of productivity is demanded.
Third, close contact or good adhesive property between the dielectric film and the solid electrolyte is required. If the adhesive property is poor, the product deteriorates or the uniformity is lost in the production, as a result, the production yield decreases or the durability in use has a problem.
In order to solve these problems, the electrically conducting polymer such as polypyrrole is electrolytically or chemically polymerized and the polymer obtained is used for the solid electrolyte of a solid electrolytic capacitor in the above-described methods. However, capacitors obtained are not satisfied in the uniformity of the electrically conducting polymer layer and properties as an electrolytic capacitor such as soldering heat resistance and impedance properties are not satisfactory.
For the electrical conducting layer used to join the cathode lead terminal and the solid electrolyte layer, an electrically conducting paste comprising an electrically conducting filler and a synthetic resin binder is usually used. A metal powder such as gold, silver, copper, etc., and carbon powder are generally used for an electrically conducting filler. The synthetic resin usually used includes epoxy resin, phenol resin and the like. Besides these resins, polyamide or polyimide resin, fluororesin (JP-A-5-152171) and acrylic resin (JP-A-7-233298) are also known.
Conducting carbon pastes have been used as die-bonding materials serving as adhesives between a silicon chip and a lead frame, or in a conducting paste layer of a solid electrolytic capacitor. In addition, a conducting paste containing a fluorine-containing polymer serving as a binder resin is also proposed (JP-A-2-5304). Such conducting pastes for die-bonding are demanded to have high conductivity, high heat resistance, low contraction stress generated during die bonding, and low water absorption ratio after die bonding. In addition, during heating conjunction, the paste must have ability to reduce stress generated between a silicon chip and a lead frame.
However, silver pastes using a common synthetic resin for the binder are high in the modulus of elasticity and a high stress is often generated by the reflow and the like, which causes increase in the leakage current or heat deterioration of the impedance due to separation at the paste interface. Furthermore, this kind of paste has high water absorption, accordingly, the performance thereof is liable to deteriorate in high temperature and high humidity conditions.
Some silver pastes use fluororesin as the binder but these pastes have also high modulus of elasticity and a high stress is generated by the reflow or the like to cause defect.
Silver, which is excellent in cost and performance, is widely used as a conducting material. However, due to unfavorable migration phenomenon of silver, when used as a paste of a solid electrolytic capacitor, a conducting silver paste is often used only after an electrically conducting carbon paste is applied.
There are many proposals regarding a conducting material, a binder, and a solvent used in an electrically conducting carbon paste. For example, JP-A-9-31402 discloses combination of natural graphite (flake graphite having a size of 10–20 μm) and carbon black, serving as a conducting material. JP-A-5-7078 discloses a carbon powder having projections and serving as a conducting material. JP-A-4-181607 discloses the combination of carbon black having a size of 20 μm or less and a synthetic resin, serving as a combination of a conducting material and a binder. JP-A-7-262822 discloses the combination of a flake graphite powder and a micro-graphite powder (aspect ratio: 10 or more, average particle size: 10 μm or less) and an epoxy resin, serving as a combination of a conducting material and a binder. JP-A-61-69853 discloses the combination of graphite and a fluorine-containing polymer (e.g., PTFE micro-particles), serving as a combination of a conducting material and a binder. JP-A-4-177802 discloses the combination of a carbon powder and a glycidyl ether, serving as a combination of a conducting material and a solvent. Furthermore, a number of synthetic resins serving as a binder, such as polyethylene, epoxy resins, and phenolic resins, are proposed.
However, an electrically conducting carbon paste produced from natural graphite has a disadvantage of low conductivity, since natural graphite has flake form and therefore attains poor packing, and contains a large amount of impurities. In addition, when the paste of this type is applied, peeling of the paste tends to occur at the interface thereof since the surface thereof has low irregularity, and the paste has a problem that the heat deterioration of impedance tends to occur.
Meanwhile, an electrically conducting carbon paste produced from carbon black contains very small particles, and thus enhanced packing cannot be obtained and conductivity of the paste is difficult to increase in the same way as the paste produced from natural graphite. These natural graphite-type and carbon black-type conducting carbon pastes must be subjected to dispersion treatment during paste preparation.
Employment of an epoxy resin serving as a binder provides some advantages, including low cost and easy handling. However, the epoxy resin has some drawbacks, including high rigidity, and low capacity of relaxation in response to reduction of stress generated between a chip and a lead frame during heating treatment such as reflow soldering in accordance with increase in chip size. In addition, the resin has high water-absorption ability, and thus deterioration of moisture resistance tends to occur.